One of the simplest cognitive acts one can perform is to attend selectively and fixate a portion of the visual scene. During attentive fixation, the image of the visual field is not motionless on the retina, but is subject to small movements that vary considerably depending upon the degree of head and body support. However, these small movements do not appreciably affect perceptual performance. A fundamental question in visual neurophysiology is how the visual system takes this positional instability into account during its processing of the elemental features of the visual scene. The broad objectives of this research program are to elucidate the central neural mechanisms of attentive processes that underlie the human's capacity to segregate and group elemental visual features into the figure and background combinations of object perception. The studies proposed in this application address a specific issue of this broader program,namely whether or not primary visual cortex solves the instability problem by a dynamic positional compensation process driven by retinal image motion, and if so, under what conditions. This issue will be investigated in four stages, posed below as questions, by examining single neuron activity in striate visual cortex while a subject performs a visual attentive task. First, Monocular Positional Compensation. Does attentive fixation result in a dynamic spatial stabilization of the relationship between striate cortical receptive fields and their mapping to the external world? Second, Compensation - Necessary Conditions. What visual conditions are necessary and sufficient for monocular positional compensation? Third, Binocular Compensation in Stereopsis.. Are stereosensitive striate cortical neurons insensitive to the disparate changes in eye position occurring during attentive fixation? Fourth, Compensation and Short Range Processes in Apparent Motion. Does positional compensation for low speed moving stimuli result from a predictive spatial interpolation related to short range processes of motion?